Michelle, I think you really have a point there.
It was hard for Willow, growing up as the School Nerd. Not to mention her mom seemed to have all these specific things she wanted from her daughter. For Xander to befriend Willow meant the world to her, and because of that it seemed only natural that she would fall in love with him. Xander, being male and oblivious, never noticed that. And as Willow herself said in Surprise/Innocence, Xander would rather be with someone he hated than with her, and that really has to sting. Not only did he reject her for someone else, but he rejected her for the girl that he and Willow despised, and who likewise despised them.
From then on she went out with Oz, but the idea of feeling inferior was probably there constantly. After all, Oz was the first person to take an interest in her, and so it would be only natural for her to start to wonder if he was just with her because he felt sorry for her. Add to that her delving into magic, and Giles' clear disapproval thereof. Until that point, she had been Giles' shining star as it were... she studied hard, did well in school, was great at research, loved the library... 😉
Oz then left her, she ended up meeting Tara, Tara got her mind ripped from her, then she got it back, but then they broke up for awhile... There was one thing that Willow had to hold onto when Tara left... the possibility that Tara would come back. It may seem preposterous to some, but that was probably the one thing that held Willow together as much as it did. For someone who has spent her whole life trying to be accepted... the death of her loved one was the final blow in many ways.
In contrast, Tara, though she had a hard life in many, many ways, had learned and applied the concept of contentment. Out of all the Scoobies, she was the one who most thoroughly understood that life isn't always going to go your way, and that you need to learn to be content with whatever life sends you. It may suck, but it will get better, and you have a responsibility to those you care about (dead or alive), to realize things can always be worse, and that you can't control everything and make sure it all has a happy ending. Sometimes things don't. But what matters most is how you react. Are you going to blame the universe, or are you going to remain strong in spite of adverse circumstances?
Because of this, we would never see a dark Tara, and she certainly wouldn't try to make Warren pay. She, out of everyone present, would also be the first, imo, to realize that Willow's death was accidental. (Well, purposeful in a way, but Xander's appearance freaked Warren, he fired wildly with the intent to hit Buffy, and we all know what happened then)